The present invention concerns methods and arrangements for evaluating originals to be copied, e.g., photographic negatives to be printed, with respect to copiability, and most especially with respect to image sharpness. In systems of the type in question, photoelectric means are employed to sense light transmitted through or reflected from an original, or to sense a focussed image of part or all of the original projected onto the photoelectric means.
In present-day photographic processing installations, use is made of systems which not only automatically or semiautomatically evaluate originals with respect to density and color and then implement exposure or printing corrections, but also systems which evaluate originals, e.g., negatives to be printed, with respect to image sharpness, for example so that negatives which were exposed at totally wrong distance settings or with the camera obviously in motion during the course of a still exposure can be skipped during printing.
For example, commonly owned U.S. Pat. No. 3,981,579, granted Sept. 21, 1976, to Weinert et al, discloses a system which evaluates the rate of change of density across the surface of an original in order to be able to ascertain the presence of closely adjoining extremely small regions of the original having respective densities differing from each other by more than a predetermined value. In that patent, the presence of such immediately adjoining density-difference values are presumed to evidence light-dark image transitions attributable to a sharply focussed image. The presence of immediately adjoining extreme density differences is ascertained by effecting relative motion between the original and a photodetector, to yield a scanning signal, forming the time derivative of the scanning signal, and ascertaining when the absolute magnitude of the time derivative exceeds a predetermined value, or in other ways likewise disclosed in that patent. The number of sufficiently extreme immediately adjacent density differences is then summed, and printing of the scanned negative is permitted or not depending upon whether the summed number does or does not exceed a predetermined number.
Image-sharpness evaluating systems of that type evaluate image sharpness with a degree of accuracy dependent upon how small a surface-area element of the original can be instantaneously scanned. Accordingly, consideration has been given to the use, instead of the photodetector elements of that patent, of vidicon scanning systems, inasmuch as these can provide a scanning spot of small surface area. However, such vidicon scanning systems tend to be limited to use where the original to be evaluated (which is typically one original in a long strip of transported originals) is at complete standstill during the scanning operation, or at least can be transported at relatively low speed during the scanning operation. Furthermore, vidicon scanning systems, and the like, tend to be complex and costly.